REALIZING GRAPHENE’S FULL POTENTIAL

Four Properties Are Critical to Unleashing this Super Material

By Liam Critchley* (chemistry and nanotechnology writer)

Graphene has been hailed as the ‘wonder material’ that will soon disrupt industries and usher in a new, green industrial revolution by disrupting a wide swathe of industries, transforming thousands of products and cutting hundreds of millions of tons of CO₂ emissions. Tens of thousands of products can be transformed by, to name just a few, lightweighting concrete, composites and metals used in everything from building infrastructure to electric vehicles and airplane fuselages; significantly increasing performance of rechargeable batteries; improving quantum computer efficiency; filtering seawater into potable water; and, replacing copper for electricity transmission. Graphene has a superlative combination of properties – 200 times stronger than steel, 1,000,000 times the conductivity of copper, 100 times the electron mobility of silicon and the ability to lightweight products while adding strength, flexibility and durability. Economist and tech futurist George Gilder predicts that by 2033, graphene will have an $11.5 trillion macro-economic impact.

However, instead of titanic transformation there has largely been disappointment because of a wide gap between graphene’s promise and actual performance improvements.

The root cause of this disappointment is an obsessive focus on conceptualizing graphene only as a single atomic layer of hexagonal carbon to the exclusion of the other three critical physical properties needed to realize the combination of graphene’s fantastic qualities. The optimal conductivity, tensile strength and performance of graphene as an additive in applications depends simultaneously on four physical characteristics: thinness (number of layers), lateral size, surface area and degree of defects (such as holes or bound oxygen atoms).

To realize the combination of real graphene’s outstanding properties as an additive material, it must be 1-5 atomic layers thin, almost defect free (especially free of carbon-oxygen bonds, layer breakage or ‘holes’, and other residual contaminants), and have a lateral size and surface area large enough to confer strength and high conductivity (both electrically and thermally) in the intended application. The optimal form of real graphene is large, thin and almost defect free (LTDF) graphene flakes.

This article reviews different materials marketed by hundreds of companies as graphene. What emerges is that by focusing on only the number of atomic layers, the performance of different materials varies widely and virtually all fail to deliver the properties that gives LTDF graphene its “wow factor”. As will be shown, the range of graphene materials falls along a continuum, with graphite at the lowest point and LTDF graphene flakes delivering peak performance.

Graphite

Graphite is typically millions of 2D graphene layers held together by van der Waals forces. The bonds within each graphene layer are incredibly strong while the van der Waals bonds between the graphene layers are substantially weaker. Graphite is commonly used in a wide range of products, such as pencils, lubricants, crucibles, foundry facings, polishes, arc lamps, batteries, brushes for electric motors and nuclear reactor cores.

Perhaps the most important form of graphite is a highly processed material called spherical graphite (SpG). Used in lithium-ion batteries anodes, stationary energy storage and environmental protection, almost 80% of the global supply of SpG is produced in China and has been designated a critical material by the Biden Administration. SpG is well suited to battery anodes because its round shape results in better packing, conferring higher energy density, longer life and more active sites to intercalate during charge and discharge cycles.  SpG is currently a critical component in anode manufacturing. It represents just under 50% of the material in typical lithium-ion batteries and accounts for 28% of an electric vehicle battery’s weight.

Graphene Oxide

Graphene oxide (GO) is manufactured by separating (i.e., exfoliating) graphite’s existing graphene layers. GO is similar to real graphene in basic hexagonal structure and number of atomic layers but, instead of being >98% pure carbon, it contains up to 50% oxygen-based defects on its surface and edges with a range of hydroxyl (―OH), alkoxy (C―O―C), carbonyl (C=O) and carboxylic acid (―COOH) functional groups. Graphite is chemically oxidized using strong acids (sulfuric or nitric acid) and strong oxidizers before being exfoliated via mechanical, sonic, chemical, thermal or photo-thermal methods. Because of oxidation, as well as the harsh exfoliation methods, GO tends to be <2 micrometre or nanometre in lateral flake size.

GO’s properties vastly differ from those of real graphene. The oxygen bonded to the surface of the graphene oxide layers helps separate them from graphite but interferes with the conductive and strong network of C=C bonds. GO has an electrical resistivity of ~1000 Ω/sq. and an electrical conductivity on the order of 10-3 MS/M. It also has very low thermal conductivity; while the values vary, a lot of GO materials exhibit thermal conductivities in the 0.5–1 W m-1 K-1 range. Additionally, GO is hydrophilic – meaning it dissolves in water (the degree of solubility depends “mostly on the chemical interactions at the GO/solvent interface”). GO has been used as a filtration membrane but apparently requires capturing the GO after treatment due to secondary contamination. Further, the University of Singapore’s Centre for Advanced 2D Materials raised the concern that impurities introduced during GO processing could render it toxic for water filtration and bio-incompatible.

With their defects and small flake size, GO materials are considered very low quality compared to LTDF graphene. GO also does not come close to delivering the tensile strength and conductivity of LTDF graphene. Another key difference is that real LTDF graphene is a hydrophobic material, whereas GO is hydrophilic.

GO is relatively inexpensive to manufacture but also uses environmentally unfriendly harsh chemicals. The “sweet spot” for GO’s use appears to be for applications demanding large quantities at the lowest possible price point, such as an additive to ordinary concrete, asphalt and some composites in applications that do not require high performance, as well as for barrier coatings and as a filler material. There is on-going research about how to utilize the functional group defects to bond other functional groups for very specific applications.

Reduced Graphene Oxide

Reduced graphene oxide (rGO) is a somewhat purified form of GO with a hexagonal carbon backbone and varying levels of oxygen functional groups. Once the graphene layers have been exfoliated from graphite into GO, the idea is that by removing the oxygen from GO, the product will be closer to real graphene. The purification (or reduction) process does not increase but may decrease GO’s already small flake size. One of the primary downsides of rGO is the reducing process creates even more structural defects than other graphene materials.

The defects are in the lattice regions where the oxygen groups were removed using very harsh chemicals, such as hydrazine or sodium borohydride. These defects inhibit rGO’s mechanical and electrical performance.

However, because rGO has a lower oxygen content (10%-15%) than GO, its properties are somewhere between GO and real graphene, although far below LTDF graphene. rGO tends to be more hydrophobic than GO. It has decent electrical conductivity of a few MS/M. Its thermal conductivity varies hugely ― from 30 to 2600 W m^-1 K^-1― based on the flake size, number of layers and amount of residual oxygen defects.

rGO was historically seen as a cheaper route to producing a usable form of graphene. It is often chosen over GO because it has better properties. However, graphene nanoplatelets have emerged as a less expensive alternative to rGO as they do not use many of the harsh chemicals required for GO synthesis followed by reduction to rGO. rGO is still used in lower value-in-use applications because what it lacks in performance relative to few-layer graphene, it makes up for in lower cost.

Nanoplatelets

Nanoplatelets have been variously defined by manufacturers, distributors, researchers and journalists. Essentially, nanoplatelets are stacks of nanometre-sized graphene particles shaped liked platelets that are between 11-100 atomic layers thick and hugely varying lateral size. Nanoplatelets are essentially thin stacks of graphite and are commonly made by mechanically pulverizing mined graphite. Due to the many stacked layers, the surface area of nanoplatelets is much lower than that of LTDF graphene – think about picking up one sheet of paper, versus a stack of 50 sheets. These products are variously described as nanoplatelets, graphene nanoplatelets (GNPs) and exfoliated graphite nano-platelets (xGnP). This material has even been marketed as “pristine graphene” based on having fewer defects than rGO. But this is an oxymoron because the definitions of graphene and nanoplatelets are mutually exclusive (graphene is 1-10 layers, while nanoplatelets are 11-100 layers).

While nanoplatelets are made of hexagonal carbon, their properties differ substantially from LTDF graphene because the number of layers makes a critical difference. Nanoplatelet properties are a middle ground between graphite and real graphene, with the loss of high-performance properties that characterize real graphene. They can contain fewer defects than rGO, but they are not at the quality level of real graphene as the mechanical pulverization causes holes and breaks in the platelets.

In the marketplace, nanoplatelets have a wide range of product variability. For instance, nanoplatelets can be anywhere from a few nanometres all the way to 100 nanometres in thickness. In contrast, real graphene’s thickness is consistently <3.4nm but preferably <1.675nm. Not only that, but the platelet lateral size also varies wildly from company to company and sometimes from batch to batch from the same company.

Nanoplatelets share a number of characteristics with real graphene, but their properties provide substantially less performance. For example, nanoplatelets have some level of thermal conductivity, depending upon the number of atomic layers and platelet size (ranging from ~3000 W m-1 k-1 to a low of 1000 W m-1 K-1).  The same goes for electrical conductivity, which is dramatically lower than real graphene. Nanoplatelets also tend to be less flexible than other graphene materials – and especially LTDF graphene – because the greater the number of atomic layers, the more rigid and brittle the nanoplatelet.

Because nanoplatelets are relatively inexpensive to produce, they are used in a number of applications, including barrier/protective composites, conductive coatings, as a reinforcement additive in polymer and mixed metal composites, as filler materials for introducing flame retardancy into composites, in sensors, and for making battery electrodes more robust. As with GO, nanoplatelets are increasingly used in road pavement throughout Europe.

Nanoplatelets, GO and rGO are at the heart of the confusion and disillusionment with real graphene. Because nanoplatelets are sometimes marketed as graphene, companies buying nanoplatelets expected to realize the full range of graphene’s superlative qualities when added to their existing products. Instead, these materials underdeliver.

Graphene Nanoparticles/Graphene Powders

Graphene nanoparticles (GNs) (also referred to as graphene powders) are by far the fastest growing segment of graphene materials in the world today. GN refers to graphene that is 500nm in lateral size or smaller (as low as 1nm). The bulk of these materials are manufactured by converting a hydrocarbon such as methane into nanoparticles, typically 15nm-200nm lateral size, but other feedstocks are gaining traction, such as trash, biomass and metallurgical coke.

GNs have been touted in a range of applications, but they appear to be a material in search of a cost-effective use because the extremely small lateral size of GNs limits its conductivity and strength compared to LTDF graphene flakes (depending upon the application).

A growing number of companies are producing GN’s, such as:

• UK company Levidan uses a plasma cracking process to turn methane into graphene nanoparticles and hydrogen. The resulting graphene produced is around 200nm in lateral size.
• US company Rimere also uses plasma technology to crack methane to produce hydrogen and graphene nanoparticles.
• Canada’s Universal Matter uses the Flash Joule method, developed at Rice University by Prof. James Tour’s lab. This involves a high current and high voltage pulse being applied to carbon materials for a hundred milliseconds. The resulting “turbostratic” graphene ranges from 13nm-1.2µm in size.
• US company Hydrograph Clean Power uses detonation to break apart carbon-bearing gas (acetylene), producing turbostratic graphene nanoparticles in the 20nm-50nm lateral size range.

Most graphene nanoparticles are stacked aggregates of graphene layers that need to be exfoliated and dispersed prior to use and have at least 2 orders of magnitude lower surface area compared to LTDF graphene.

3D Graphene

Lyten, Inc. is another company that converts methane into graphene. Lyten began by manufacturing graphene “sheets” (which is a different form factor from graphene flakes). With a limited market for graphene sheets, Lyten then “crumpled” these sheets to create a form factor that could be used as an additive material, which is how graphene flakes are used. Lyten branded these crumpled sheets as 3D graphene.

Since 3D graphene is synthesized from methane, the size of each graphene particle is in the 15nm-200nm range – meaning, the same size as nanoparticles. 3D graphene typically has higher surface area than graphene nanoplatelets and graphene powders, which are important in many applications, but does not have LTDF graphene’s large lateral size and surface area.  This significantly limits 3D graphene’s tensile strength and conductivity – 3D graphene will likely have very poor composite properties due to small size and crumpled structure. Additionally, no research could be found on whether 3D graphene’s small particle size makes it susceptible to clumping, a drawback from which nanoparticles, GO and rGO can suffer.  Therefore, it is not known whether 3D graphene will create weak zones in structures or interfere with electrical or thermal conductivity.

While 3D graphene is probably better than GO in some applications, it is significantly limited compared to real graphene and, especially, LTDF graphene flakes. However, 3D graphene products might be viable for high-surface area graphene products, which cannot be realized by nanoplatelets or most graphene powders.

Large, Thin and Nearly Defect Free (LTDF) Graphene

As discussed above, the optimal combination of graphene’s superlative properties, when used as an additive to other products, can only be conferred with large, thin and nearly defect free graphene flakes. Although there are about 20 methods used to produce materials called graphene, the only company with a known technology to manufacture LTDF graphene flakes is Avadain, Inc. Avadain’s technology overcame the problems associated with the traditional electrochemical exfoliation process to separate the graphene layers of flake graphite while retaining much of its naturally occurring large flake size.

A key aspect of Avadain’s process innovation is hydrogenation, rather than oxidation. Unlike the significant defects created during the production of GO and rGO, the hydrogenation is fully reversible through thermal annealing using no harsh chemicals, resulting in few defects which enables very high conductivity. In addition, the larger average size reduces the number of bridges between flakes, which have contact resistance within the material (composite, coating, etc). Moreover, the lateral flake size is crucial for applications seeking significantly improved strength, thermal and/or electrical conductivity or chemical resistance. It has been demonstrated in studies worldwide that the larger the lateral flake size, the higher the tensile strength and electrical conductivity. In sharp contrast to other graphene materials, LTDF graphene flakes have also been shown to form more interfacial interactions in composites due to its larger surface-to-volume ratio and absence of problematic defects.

LTDF’s larger lateral size also means that it shouldn’t, in theory, aggregate as readily in a host material compared to tiny graphene flakes/nanoparticles/powders.

This is important because aggregation can cause localized property variation, as well as dead zones, with little to no filler, leading to weakness and cracking. Performance improvement can also be achieved with less volume of LTDF because the substantially larger lateral size of LTDF provides more uniform dispersion throughout the host material. Additionally, it should be noted that there is a critical length which is needed for optimal strain transfer. If the graphene lateral size is too small, the strain within the composite cannot be optimally transferred onto the graphene. Therefore, the mechanical strength of most of the graphene in the market today cannot be fully utilized.

LTDF graphene has very low density – weighing about 0.77 milligrams/meter² with an extremely high surface area. Comparatively, a square meter of paper weighs 1,000 times more. A cubic meter of plain concrete weights ~2,400 kg/m³. Most graphene used as an additive is measured in percentage weight of the end product because a lot of material is required, whereas LTDF graphene flakes – with its large surface area and few-layered thickness – requires comparatively little material and is measured by volume. In applications, substantial quantities of other graphene materials might be used versus LTDF graphene flakes. Imagine adding one sheet of paper as opposed to a stack of 50 paper sheets, and the reduction in weight that LTDF graphene presents as opposed to nanoplatelets or powders.

Finally, LTDF graphene is hydrophobic. Avadain’s process avoids the harsh chemicals required to produce GO and rGO, making it an environmentally friendly process that achieves high exfoliation yields.

There are thousands of potential high value-in-use applications for LTDF graphene flake as an additive material, including power transmission, energy storage, electric vehicles, electric aircraft, battery electrodes, polymer composites, construction materials, defense, anti-corrosion coatings, water filtration, to name a few.

A Note About CVD Graphene

Chemical vapor deposition (CVD) is used by a number of companies to manufacture graphene in the form factor of a cm or meter-scale, single sheet (as opposed to a flake or particle). Graphene sheets have limited use to confer graphene’s superlative properties to other products (with the possible exception of Lyten’s branded 3D graphene in energy storage). It is worth mention, however, because of the large CVD production capacity in the US and worldwide.

CVD graphene is built atom by atom on a substrate (most often copper foil), rather than the exfoliation of fully formed graphene layers from graphite.  The underlying substrate acts as the catalyst for the reaction, so when methane and an inert carrier gas (nitrogen, helium, etc.) is fed into the heated chamber, a reaction occurs on the catalytic surface, breaking down the methane and forming graphene in the process.

CVD graphene is mono-layer and generally formed without defects. Its cost per unit weight is orders of magnitude higher than other graphene materials, and the production scalability is generally considered lower based on the challenges of roll-to-roll processing. It’s not clear if profitable applications have been developed yet. Some current proposed uses are transparent electrodes for solar cells, liquid-crystal displays, light-emitting diodes, and touch screens, as well as in field-effect transistors (FETs), flexible electronics, sensors and wearable devices. It remains to be seen whether films of LTDF graphene will be preferred by industry in some high value uses for CVD graphene because LTDF is significantly easier to produce and process.

CVD graphene is not without its challenges, including manufacturability. It is tricky to remove, manipulate or transfer CVD graphene from the substrate on which it is formed, and the ultraclean copper catalyst film is very costly. It also requires tight process parameters which, if not rather precisely achieved, can create defects. The presence of these defects will affect the electrical and mechanical performance of the graphene sheet. Another challenge is that the CVD process uses harsh chemicals and produces toxic off gases. Finally, the form factor – a single graphene sheet – limits its use as an additive. In short, CVD graphene and LTDF graphene will likely have (at least in the foreseeable future) completely distinct applications, markets and trajectories.

Conclusion

Many have tried to define real graphene solely in terms of thinness – 10 or fewer layers of carbon atoms with each atom bound to three neighbors in a honeycomb structure. This definition encompasses a broad range of materials, including GO, rGO, 3D graphene, graphene particles/powders and LTDF graphene flakes, although it excludes nanoplatelets. However, this definition of graphene is insufficient to realize the “wonder material” that promises to disrupt industries and transform thousands of products.

Industry and government frequently report disappointment with the performance of most of these materials. This disappointment stems from the disconnect between the expected benefit of real graphene and the performance benefit of the supplied materials which are marketed as graphene only because of the number of atomic layers. As discussed above, four properties are simultaneously required to unlock all of graphene’s miraculous qualities – thinness, lateral size, surface area and level of defects.

The appropriate way to view graphene materials is along a performance-in-use continuum, with graphite at the lowest point and LTDF graphene at the peak. Each material has uses based on its cost effectiveness. The appropriate selection of graphene depends upon the value-in-use equation. For road construction, GO or nanoparticles may be most cost-effective. For sound insulation in automotive applications, nanoplatelets may be the best choice. But for high value applications where optimal strength, conductivity and/or durability are required, there is no substitute for LTDF graphene.

There can no longer be any doubt that real graphene is a wonder material poised to create a green Industrial Revolution. When added to existing materials, it has the potential to help solve some of the planet’s most pressing needs by improving strength to weight performance, electrical and thermal conductivity, chemical resistance and impermeability. The greater the need for high performance, the higher the quality of graphene along the continuum is required.

LTDF graphene delivers real graphene’s optimal performance. Some examples of its potential include:

• Substantially reducing the carbon footprint of transportation and mobility by developing lighter chassis and fuselages, more efficient electric motors, and lighter, more powerful batteries
• Shrinking the carbon footprint of building new infrastructure by slashing the amount of concrete and other construction materials (concrete is the single largest CO2 emitter, accounting for about 8% of global emissions)
• Improving the efficiency of power transmission lines without the need for heavy copper
• Boosting efficiency of solar cells, wind turbines, hydrogen fuel cells and energy storage devices to support the faster adoption of renewable energy technologies
• Creating energy efficient and cost-effective water desalination membranes to address the global scourge of water scarcity
• Reducing waste and maintenance costs from more effective coatings and lubricants by improving the efficiency and life cycles of equipment and infrastructure
• Enabling a new generation of electronics, medical devices, drug delivery and, yes, perhaps even replacing computer chips…

Once industry and government better understand the performance characteristics of different materials, graphene can finally achieve the $11.5 trillion macro-economic impact predicted by George Gilder.

* Grateful appreciation to the following individuals, listed in alphabetical order, who have contributed to this article: Dr. Julia Faeth, Dr. William Grieco, Dr. Sarah Roscher, Dr. Akansha Urade and Dr. Kevin Wyss.

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